The present invention relates generally to microtechnology and the fabrication process for developing micromechanical and microelectrical systems such as micro-actuated elements, microengines, or micromachines. More particularly, the present invention is directed to a means of fabricating a gas-driven microturbine that is capable of providing autonomous propulsion in which the rapidly moving gases are directed through a micromachined turbine to power mechanical, electrical, or electromechanical devices by direct mechanical linkage of turbo-electric generator components in a domain ranging from tenths of micrometers to thousands of micrometers. By optimally selecting monopropellants or bipropellants to be the fuel set, a more efficient gas-driven microturbine can be realized from the increased mass flow rate of the gas stream due to the higher combustion reaction energies of these fuel sets. Additionally, compressed gas can be utilized to provide a high-flow gas stream for the gas-driven microturbine. The present invention is adaptable to many defense and non-defense applications, including the provision of mechanical power for miniature devices such as fans, geared mechanisms, mechanical linkages, actuators, biomedical procedures, manufacturing, industrial, aviation, computers, safety systems, and electrical generators.
In the last decade, great interest has developed in the emerging field of microtechnology. Microminiature machines (micromachines) represent an emerging technology with significant national and international interest. Generally, these micromachines comprise the larger class of components usually referred to as microelectromechanical systems (MEMS) that are fabricated using now-standard semiconductor manufacturing techniques. The systems are integrated microdevices combining electrical and mechanical components fabricated using integrated-circuit-compatible batch-processing techniques and range in size from micrometers to millimeters. The inherent motivation for developing MEMS is the ability to perform specialized applications through smaller, faster, lighter, and more accurate micromechanical devices. These systems can control, sense, and actuate on the microscale and function individually or in arrays to generate effects on the macroscale.
The area of micromechanics deals with actuators and sensors which are on the order of micrometers. This ability results in applications which take advantage of potentially high packing densities for simple microdevices which, when combined in a system, can perform complex and precise mechanical and electrical functions. Another important aspect is found in micropositioning applications since these microdevices can be moved by small distances which can be measured or monitored accurately. Micromechanical devices are significant because they can have small moments of inertia, but they are currently limited in their abilities to generate adequate output forces and torques for specialized applications. However, the result of extensive research in micromechanics and advances in polysilicon surface-micromachining have led to the development of microscopic motors of considerably low-mass, incorporating mechanisms on silicon wafers for -a number of technological applications such as micro-sensors to detect or measure changes in pressure, acceleration, temperature, vapor, or sound. Micromechanical technology can be incorporated into automobiles to diagnose and sense engine-performance or into applications involving the deployment of air bags or into sensors that can detect air pressure in tires.
One of the earliest devices fabricated from the surface-micromachining process was a device called the resonant gate resistor. This device was disclosed in an article written by Nathanson, et al., entitled, xe2x80x9cThe Resonant Gate Resistor,xe2x80x9d IEEE, Trans. Electron Devices, Vol. ED-14, pp. 117-133, March 1967. The device consisted of a transistor with a free-standing metal cantilever beam serving as the transistor gate. Subsequent work in this area led to the development of a polysilicon surface-micromachining technique described in an article by Howe, et al., entitled, xe2x80x9cPolycrystalline Silicon Micromechanical Beams,xe2x80x9d J. Electrochem. Soc.: Solid-State Science and Technology, Vol. 103, No. 6, pp. 1420-1423, June 1983.
Working with methods of producing microelectric circuitry but optimized for producing micromechanical devices, the polysilicon surface-micromachining process generally involves etching a pattern in films supported by a silicon substrate by exposing the polysilicon through a photoresist mask. By selectively etching sacrificial layers from a multilayer sandwich of patterned polysilicon films and interleaved sacrificial oxide films and through material deposition and selective removal of these various film layers, highly specialized and unique components can be structurally fabricated. The basic process involves fabricating a single layer of mechanical polysilicon to form simple micromechanical devices. However, with just one layer of polysilicon, the mechanical structures have restricted movement through elastic members attached to the substrate and provide limited mechanical movement. Therefore, the need to fabricate more sophisticated and specialized structures necessitates the deposition of multiple layers of polysilicon to form complex mechanical structures such as sliders and self-restraining pin joints. With two layers of polysilicon deposition, it is possible to fabricate rotating entities, but the ability to harness the rotary motion produced from a gear or turbine is limited; that is, there needs to be a means to fully couple the energy produced from mechanical devices formed from a two-layer polysilicon deposition. To address this problem, a third layer of poly silicon deposition would allow a gear or turbine formed from a two-layer polysilicon deposition to be interconnected by a mechanical linkage for direct actuation of ancillary components. A discussion of the polysilicon surface-micromachining batch-fabrication process is discussed in greater detail in the articles given by J. J. Sniegowski, et al., xe2x80x9cMicrofabricated Actuators and Their Application to Optics,xe2x80x9d Proc. SPIE Miniaturized Systems with Micro-Optics and Micromachines, 2383, San Jose, Calif. 1995, pp. 46-64; and E. J. Garcia, et al., xe2x80x9cSurface Micromachines Microengine,xe2x80x9d Sensors and Actuators A, 48 (1995), pp. 203-214.
The small sizes of the micromotors and recent advances in polysiliconsurface-micromachining combine to exhibit unique and novel electromechanical characteristics that are vastly different from conventional motors. Electrostatic forces in the microdomain are found to scale more favorably than the magnetic alternatives for devices designed to micro-dimensions and the use of micro-size field-generating structures enables more intense electrostatic fields to be created. Conventional motors are typically magnetically driven but he windings and magnetizable material to make such motors make it nearly impossible to duplicate or produce on the silicon chips in the microdomain due to the inherent size limitations.
U.S. Pat. No. 5,262,695 (Kuwano et al.) discloses two possible drive systems (wired and wireless) for a micromachine. A wired system has the energy source located outside the micromachine unit. This setup allows for the ability to produce smaller machines with the drive energy supplied through a feed cable. However, the cable imposes movement and control limitations on the operation of the machine. In the case of the wireless system, machine movement is less restricted since the energy source is generally mounted on the machine but this setup increases the size and weight of the entire micromachine and impairs the contemplated function(s) of the micromachine. Kuwano proposes an electrostatic motor for use as a mechanical power generating mechanism to be mounted on a micromachine that includes a rotatable semiconductor substrate and a drive electrode disposed in proximity to the substrate. Kuwano""s invention consists of an electrostatic motor comprising a rotor and a stator fabricated from silicon or a similar semiconductor. The semiconductor substrate is doped with a specified impurity element to form electromagnetic wave-static electric converters of p-n junction. To drive the micromotor, positive and negative voltages are applied to two stator poles while the remaining stators in the machine unit are grounded or put at zero electrical potential. The positive and negative stator voltages induce opposing charges on the rotor poles nearest the stator poles, and, as the voltages are continuously alternated between the stators that are located 180 degrees apart and those that are at zero potential and the rotors, the rotor begins to spin.
U.S. Pat. No. 5,252,881 (Muller et al.) discloses a method for making a microminiature electrical motor having a rotor rotatable about a fixed hub member within a surrounding stator. In particular, the fabrication of the micromotors begins with providing a substrate material with a first layer of silicon dioxide covered by a layer of silicon nitride. Next, a first layer of sacrificial material is provided on said substrate. The first structural layer over said sacrificial material is then realized by patterning and then etching said first structural layer to form the rotor and stator components. A second layer of sacrificial material is then deposited over the first structural layer in which the pattern set involves the formation of an anchor opening in the substrate at the center of the rotor. To form the hub member in said anchor opening, a second structural layer is patterned to form a flange for said hub member. The sacrificial layers are etched to separate the rotor and stator components, as well as the rotor from the hub member, so that the rotor rotates about the hub member. The invention also includes an ancillary method for protecting metallized elements in the motor circuit during the required etching steps for removal of sacrificial layers.
U.S. Pat. No. 5,043,043 (Howe et al.) discloses an electrostatic micromotor that employs a side drive design. In particular, three fabrication processes enable the formation of a moveable member in the plane of operation of the stator and spaced apart from the stator by a micron amount. The first fabrication process deposits and patterns a structural layer to form the stator and moveable member over a sacrificial layer. The second fabrication process etches channels in a first structural layer to outline a stator, a moveable member, and if desired, a bearing. A substrate is then connected to the side of the structural layer through which the channels are etched, and the opposite side is ground down to the ends of the channels to form salient stator, rotor and, if desired, bearing structure. The third fabrication process grows a sacrificial layer by local oxidation in an etched cavity of the substrate. A structural layer is then deposited and patterned over the substrate and sacrificial layer to form the stator and moveable member in a common plane.
The inventions of Kuwano, Muller, and Howe are limited in the sense that, because of the electrostatic motor design and configuration, it is difficult to direct the rotary motion produced by the rotors to drive a mechanical component or entity such as a diaphragm, slider, spring, cantilevered beam, or gear. Other problems associated with a typical drive system for an electrostatic motor include: (1) the need to incorporate complex integrated circuitry to withstand a high voltage supply (typically 100 V) to produce the rotary motion from the rotor and stator combination, (2) the need to minimize frictional losses without compromising rotor-torque characteristics, and (3) the need to increase the operating range and movement of the machine unit. Therefore, to overcome these existing limitations of the wire and wireless drivers for these micromachine systems, it is the object of the present invention to fulfill the need to find a less complicated and more practical alternative to autonomously power micromechanical and electromechanical systems. The need for an autonomous generator system has only been addressed with cursory ideas as to how to accomplish it and no attempt thus far has been made to reduce such a system to practice. Furthermore, most of the effort in micromachining technology thus far has been in the development of microscale sensors and not in the creation of an autonomous power system. The size of the sensors or actuators can become irrelevant if they are subject to power supplies of six orders of magnitude larger or more.
In view of the above-described needs and to overcome the shortcomings of the prior art, it is an object of the present invention to provide a gas-driven microturbine and a method for fabricating a gas-driven microturbine on a silicon substrate base.
It is another object of the present invention that the entire gas-driven microturbine is made primarily of polysilicon with intervening coatings of silicon nitride for electrical isolation on a single substrate using a three-layer polysilicon surface-micromachining batch-fabrication process.
It is still another object of the present invention that the gas-driven microturbine comprises a core propulsion system with a plurality of components including the holding tank, fuel set, fuel delivery system, reaction chamber, flow channel, turbine housing, turbine, turbine shroud, exhaust port, and mechanical linkage arm, all of which are fabricated using polysilicon surface-micromachining techniques on a silicon substrate.
It is still a further object of the invention that the components of the propulsion system do not result from an assembly of separately fabricated individual parts, but are fully batch-fabricated.
It is still another object of the invention that the fabrication of the core propulsion system of the gas-driven microturbine requires four depositions of polysilicon in which the first layer of polysilicon serves as the voltage reference plane and the electrical interconnect while the three remaining polysilicon layers serve to form the mechanical and structural elements of the propulsion system.
It is still even a further object of the invention that the thermal assist elements comprise a plurality of polysilicon filaments that are located internally within the reaction chamber, wherein the reaction chamber is fabricated with at least one inlet tube to provide a delivery system for transporting the fuel set from the holding tank to the reaction chamber by capillary action, pressure feed, or mechanical pump.
It is another object of the present invention that the reaction chamber can be configured to include a pre-heater element for initiating thermal decomposition of a fuel set or to provide a continuous source of heat if water is selected as the primary source for generating a high-flow gas stream.
It is yet another object of the invention that the thermal assist elements are heated from an external power source through the voltage reference plane of the present invention.
It is a further object of the present invention that the central flanged hub and hub anchor for the turbine are formed from the second deposition of polysilicon and oxide and that the mechanical linkage arm is formed from the deposition of all three mechanical polysilicon films.
It is still a further object of the invention that post-deposition anneals are performed after the second structural and third structural polysilicon depositions to ensure that the polysilicon mechanical films do not exhibit undesired internal stress which would cause deformation of the final structural layer.
It is a final object of the present invention that the entire gas-driven microturbine assembly is subject to a hydrofluoric acid dip to release the free-standing components of the gas-driven microturbine.
Other objects, advantages and features of the invention will become apparent from the following detailed disclosure of embodiments thereof, presented in conjunction with the accompanying drawings.
The present invention comprises a gas-driven microturbine capable of micropropulsion to autonomously power miniature systems such as a turbine for effectuating mechanical loads by mechanically linking the turbine to an actuated element to generate power in the microdomain. Depending upon the particular application, the fuel set of the present invention is capable of generating large volumes of gaseous products or heat where the gas stream is routed through a flow channel and enters directly into a turbine housing. Applications requiring higher mass flow rates can utilize liquid agents such as conventional monopropellants (hydrogen peroxide or hydrazine) or bi-propellants to produce a more efficient gas-driven microturbine.
The invention further comprises a method for fabricating a gas-driven microturbine with a plurality of core propulsion elements utilizing a fuel set (steam, monopropellants, bipropellants, or compressed gas). In operation, the fuel set is injected into a holding tank, transported to a reaction chamber (by capillary action, mechanical pump, or pressure feed), and passed through at least one inlet tube (but typically multiple inlet pipes) which directs the generated large volume of gas and heat through a flow channel and across the turbine blades located within a turbine housing. The high flow gas stream entering the turbine shroud from the flow channel drives a turbine which acts on an element (hereinafter referred to as the actuated element) that is connected to the turbine by a mechanical linkage arm or direct gear coupling. Since the turbine entity is mechanically linked to the actuated element, the rotary motion of the turbine induces linear motion in a mechanical linkage arm connected to the turbine by pin joints where the mechanical linkage arm drives a mechanical load. The gas stream is then discharged to ambient through an exhaust port which extends from the turbine housing.
In the case where the fuel set employed undergoes a phase change, a liquid reservoir provides a continuous gaseous head of suitable pressure. If the fuel set involves a chemical reaction, the liquid reactants are transported to the reaction chamber by capillary pumping, pressure feed, or mechanical pump, whereby the reaction produces the high-flow gas. The reaction may also be spontaneous or initiated by heat or a catalyst.
The gas-driven microturbine is fully batch-fabricated and not an assembly of separately fabricated piece-components of the prior art. The inclusion of a third deposited layer of mechanical polysilicon allows for greater complexity in the micromechanical design. The first polysilicon layer is referred to as xe2x80x9cPOLY 0xe2x80x9d and does not form mechanical structures but acts as electrical interconnect and shield polysilicon on the silicon substrate. The first, second, and third mechanical polysilicon films are referred to as xe2x80x9cPOLY 1,xe2x80x9d xe2x80x9cPOLY 2,xe2x80x9d and xe2x80x9cPOLY 3,xe2x80x9d respectively, and these films are where the mechanical structures of the gas-driven microturbine are created.
The gas-driven microturbine and its micropropulsion system components are fabricated from a series of structural layers using a number of sequentially deposited films of fine-grained polycrystalline silicon and silicon nitride, with interleaving sacrificial layers of silicon dioxide, on a single-crystal silicon substrate material. The fabrication process starts with a single crystal silicon substrate that is coated with a dielectric isolation film of Low Pressure Chemical Vapor Deposited (LPCVD) silicon-rich nitride over a thermal oxide. The structural components are realized through an intricate and delicate patterning, chemical etching, deposition, and removal process from a multilayer sandwich of polysilicon films and interleaving sacrificial films. Isolation films (the interleaving sacrificial films) ensure that proper electrical isolation is achieved between electrically active parts of the gas-driven microturbine structure. The first patterned layer serves as the electrical interconnect and shield polysilicon, POLY 0. Next, the stator-to-substrate anchor areas are created (POLY 1), followed by the deposition of a first sacrificial layer, after which stiction-reduction dimple molds are patterned into the first sacrificial layer. A subsequent polysilicon and oxide deposition fills the anchor and mold areas for attaching the mechanical structures to the substrate, after which subsequent polysilicon and oxide depositions and selective removal of sacrificial layers form the core propulsion components of the gas-driven microturbine.
Let it be understood that the foregoing description is only illustrative of the invention. To those skilled in the art to which this invention relates, many changes in the construction and widely different embodiments and applications of the invention will suggest themselves from the spirit and scope of the invention. The disclosure and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. Therefore, the objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations addressed in the appended claims.